The present invention relates generally to single phase alternating current induction type electric motors and more particularly to such motors especially designed for use in hermetically sealed compressors and adapted to operate at several voltages without need of moving contact devices.
In certain hermetically sealed compressor applications for such use in connection with refrigeration, air conditioning and the like it is necessary to design single phase A.C. electric motors, that is the stator and rotor components of the compressor for operation at several dissimilar voltages, by way of illustration, 208/230 volts or 220/240 volts. These motors usually utilize a main winding and a start winding displaced in phase from the main winding, with both windings being excited under starting conditions and the start winding being deenergized after the motor has attained so-called running speed, by current sensing or voltage relays or other switches having moving contacts responsive to motor operating conditions disposed externally of the compressor casing. Such external relay with the movable contacts require additional electrical leads from the compressor, are generally noise producing when operating, and are somewhat costly to utilize, among other things.
In an attempt to circumvent the use of external relays, it has been suggested that some solid state arrangement be employed such as the one identified as PRIOR ART in FIG. 1. In this approach a main winding 11 is carried by a slotted stator (not shown) to produce when excited in concentrically disposed coil groups a and b across power terminals L1, L2 a number of primary magnetic poles, such as two poles of opposite polarity in the exemplification. A start winding 12, displaced ninety electrical degrees from the main winding, includes the same number of coil groups, c and d, as those of the main winding and has one of its ends connected to the main winding interpole connection 13. A solid state device mounted externally of the compressor in the form of a positive temperature coefficient resistor 14, a heat responsive current limiting device, and a capacitor 16 are connected in shunt relation to each other between L2 and the other end of the start winding 12. During starting conditions when device 14 is at a low temperature its resistance is such that it permits applied power to by-pass capacitor 16 to energize start winding 12. At a predetermined transition temperature, the resistance of device 14 increases abruptly, usually chosen to coincide with the running speed of the motor, and power is applied through the capacitor 16 but device 14 is operative to reduce substantially the effectiveness of the start winding 12 during running conditions. With this arrangement the main winding is used similar to an auto transformer to power the start winding circuit including device 14 at approximately half voltage. Unfortunately, the main winding poles "a" and "b" carry different net currents and a slight change in current phases which create spatial harmonics in the air gap magnetomotive forces referred to as an unbalanced MMF main polar flux pattern. This in turn seriously affects the torque curve, resulting in a so-called "dip" in the curve or loss of torque as the motor comes up to speed as well as producing side pull forces on the rotor. Side pull adversely affects bearing life and operating noise of the motor. Any attempt to increase the start winding current to compensate for lost starting torque will also increase the unbalance in the main winding poles which in turn increases the adverse dip and side pull characteristics of the motor. These latter motor characteristics could if great enough create failure to start, operating noise, and premature bearing failure problems. Thus, it will be appreciated that this approach has not been entirely satisfactory for one reason or another even though it does employ the desirable solid state device rather than an externally mounted switch having movable contacts to remove the start winding from the winding circuit during running conditions.